The present invention relates, in general, to well logging methods useful in petroleum exploration, and more particularly to an improved, highly sensitive, down hole magnetometer for detecting field perturbations caused by geologic structures in the vicinity of a well bore.
Geophysical well logging methods are extremely important in the exploration for petroleum deposits and for the systematic development of an oil field for production after it has been located. Historically, electrical conductivity and self potential measurements for these purposes have been important, and over the years numerous patents have issued on such techniques and the apparatus for carrying them out. Typical examples of the approaches taken in the prior art are found in U.S. Pat. No. 2,359,894 to Brown et al, U.S. Pat. No. 2,723,274 to Williams, and U.S. Pat. No. 3,697,864 to Runge. In Brown et al, electrodes are embedded in the earth or are placed in boreholes spaced from a main borehole in which is placed a measuring unit responsive to the current flow between the electrodes. In the Williams patent, current injection is accomplished by electrodes which may be suspended in the main borehole a predetermined distance from the measuring unit, which is also suspended in the borehole. As an alternative, the patent discloses the use of surface electrodes for current injection. The Runge patent discloses a plurality of current electrodes and a plurality of potential measuring electrodes secured at spaced locations along a logging cable or to a drill string within a well bore. In each of the foregoing patents, the current injected into the geological formations surrounding the borehole is either DC or very low frequency AC, and variations in the electrical current flow due to variations in the conductivity or resistivity of the geological formations are measured. Such measurements may be made by way of current or voltage sensitive electrodes, by induction coils, or the like.
The patterns of current flow and the variations in potential which exist within the geological formations surrounding the borehole have a direction and magnitude at any given location which reflect the electrical conductivity or resistivity of the stratum in that area. By measuring and recording these variations, it is possible to determine the general characteristics of the formations, and to identify anomalies which would indicate the probable locations and dimensions of mineral beds, petroleum bearing deposits, and the like.
However, prior systems for the logging of geological formations have been found to be of limited usefulness, since they lack the sensitivity required to make meaningful measurements at any appreciable distance from the current or voltage source. Thus, the measurements have been limited to relatively short lateral distances from the borehole, or where the source electrodes are located at the surface of the earth, at relatively shallow depths. As a result, it has been difficult to obtain accurate measurements of deposits which are not actually penetrated by the borehole, and even where the borehole passes through or very close to the edge of a deposit, it has been difficult to determine with any degree of certainty the dimensions of the deposit. Thus, although a deposit may disturb a current flow pattern, the lack of sensitivity in prior measuring devices has made it difficult, if not impossible, to determine the nature of the geological anomaly any appreciable distance from the point of measurement.
Attempts have been made to solve this problem through the provision of more sensitive detectors, and to this end magnetometers sensitive to faint magnetic fields have been proposed. However, in order to obtain accurate measurements of subsurface magnetic fields, the horizontal components of the fields must be detected, and in prior devices this required a horizontally oriented magnetometer having significant horizontal dimensions. Unfortunately, exploratory well bores have a diameter of between about 4 and 6 inches, and simply do not provide the space needed for conventional magnetometers of the required sensitivity. In order to accommodate prior magnetometers, then, it was often necessary to redrill existing exploratory wells, or to drill new exploratory wells with much greater diameters. This not only was prohibitively expensive, but did not result in significantly better results.